This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-365573 filed on Nov. 30, 2000.
1. Field of the Invention
The present invention relates to a valve timing adjusting device which can perform continuously variable control of the phase of the opening-closing timing of an intake valve or an exhaust valve driven by a camshaft of an internal combustion engine and, more particularly, to a hydraulic vane-type continuously variable valve timing system.
2. Description of Related Art
In general, vane-type continuously variable valve timing adjusting devices which can perform continuously variable control of the phase of intake or exhaust valve timing of an internal combustion engine are known. The variable control is carried out in accordance with a phase difference caused by relative rotation between a timing chain and a chain sprocket by driving a camshaft through a timing pulley and the chain sprocket which rotate in synchronization with a crankshaft of an internal combustion engine.
The vane-type continuously variable valve timing system is provided with a hydraulic servo system such as an advance hydraulic chamber and a retard hydraulic chamber in the inner peripheral wall of a timing pulley. The servo system causes the hydraulic rotation of a vane rotor as one body with a camshaft to the advance side or the retard side, thereby changing the phase of the intake or exhaust valve opening-closing timing. An oil pump is generally adopted and driven to rotate synchronously with the engine crankshaft to produce an oil delivery proportional to the engine speed. The pump also serves as an oil pressure source for supplying the oil pressure to the advance hydraulic chamber and the retard hydraulic chamber.
When the engine is operating at a low speed, the oil delivery from the oil pump decreases. Therefore, a problem arises in that, especially at a low engine speed and at a high oil temperature, oil leakage increases due to lowered oil viscosity. This lowered oil pressure results in substantially decreased oil pressure to be supplied to, and discharged from, the advance hydraulic chamber and the retard hydraulic chamber and, accordingly, in incomplete operation of the vane rotor which has a plurality of vanes on the outer periphery. Previously, in the prior art, technology such as JP-A 11-336516 has been proposed for the purpose of improving response by a mechanism for controlling the vane oscillation during operation at a low engine speed. According to this prior art, a plunger and check valve mechanism are employed to control vane oscillation.
The oil pressure accumulated in the plunger is held by the check valve to prevent reverse rotation of the vane during oscillation when the engine is operating at a low speed. The valve timing adjusting device of the prior art, however, has the problem that, despite its simple construction, the increased number of plungers will increase the number of parts and the manufacturing cost.
At a high oil temperature, at which an improvement in phase response is required, the amount of oil leakage increases, causing the valve timing adjusting device to improperly operate under the condition that the phase response needs improvement, and accordingly no sufficient effect is achievable. Furthermore, to hold the vane in the intermediate phase, the oil pressure must be balanced. However, because the vane is loaded by the plunger which is independent of the hydraulic servo system, oil pressure balance can not be established, and accordingly the vane will be unstable in the intermediate phase.
Paying attention to changes in oil pressure in a retard hydraulic chamber which are likely to occur with vane oscillation caused by operation of an intake or exhaust valve of an internal combustion engine, it is an object of the invention to improve the response of phase conversion, especially to improve the advance response, by using a simple structure and without using a special means for preventing the vane oscillation during engine operation at a low speed and at a high oil temperature.
According to one embodiment of the invention, a communicating passage is formed to communicate with the advance hydraulic chamber and the retard hydraulic chamber, and furthermore, a valve device having a valve body in the communicating passage is provided. Thus the oil pressure supply and discharge means is controlled by utilizing changes in oil pressure in the retard hydraulic chamber at the time of an advancing operation performed with a negative torque, thus supplying oil pressure from the oil pressure source to the advance hydraulic chamber and discharging oil pressure from the retard hydraulic chamber and also moving the oil from the retard hydraulic chamber into the advance hydraulic chamber.
Therefore, even at a low engine speed and at a high oil temperature, the oil flows from the retard hydraulic chamber into the advance hydraulic chamber by the amount of advance caused by the negative torque. That is, of the vane oscillation resulting from torque variation of the camshaft, the amplitude of vane oscillation toward the advance side is utilized to allow the rotation of the vane rotor in the direction of advance. Furthermore, since the amount of oil flowing into the advance hydraulic chamber increases, the advance response can be improved by a simple structure at a low cost without providing a special means for preventing the oscillation of the vane rotor. In this case, it is advisable to adopt a check valve, as the valve device, having a valve body (a ball valve) which checks the outflow of oil from the advance hydraulic chamber to the retard hydraulic chamber.
Furthermore, a flow control valve for controlling the flow rate of oil flowing in the communicating passage in accordance with the oil pressure in the retard hydraulic chamber is provided in the communicating passage which communicates with the retard hydraulic chamber and the advance hydraulic chamber. During advancing operation when the advance hydraulic chamber communicates with the oil pressure source and the retard hydraulic chamber communicates with the drain line, the oil in the retard hydraulic chamber moves into the advance hydraulic chamber by an advance angle through which the vane rotor is advanced by the negative torque. Of the vane oscillation resulting from camshaft torque variations, the amplitude of the oscillation toward the advance side is utilized to move further in the direction of advance. Furthermore, because of a small pressure loss and an increase in the amount of oil flowing into the advance hydraulic chamber, the advance response can be improved.
Continuing, the flow control valve features closing the communicating passage when the oil pressure in the retard hydraulic chamber exceeds a specific value, and also opening the communicating passage when the oil pressure drops below the specific value. Thus, when the engine is operating at a high speed, the amount of oil delivered from the oil pressure source into the advance hydraulic chamber increases, thereby providing a sufficient oil pressure within the advance hydraulic chamber. Therefore, the flow control valve will not open, thereby providing no effect to the hydraulic servo system.
Furthermore, the timing rotor has a cylindrical shoe housing which houses a vane rotor slidably and rotatably mounted on the inner peripheral surface. Formed within the shoe housing are a plurality of approximately opposing trapezoidal shoes circumferentially arranged projecting radially around the inside diameter. On the vane rotor are provided a plurality of approximately sectoral vanes formed substantially opposite in a circumferential direction, projecting radially on the outside diameter side so that they will fit in clearances formed in the circumferential direction of the plurality of shoes. The communicating passage is provided in each shoe of the shoe housing. The communicating passage does not project out of the timing rotor, and therefore the timing rotor is very compact, requiring no special hydraulic piping and thereby reducing costs.